Not every borrower is equally likely to pay you back. A bond issued by the UK government and one issued by a struggling retail chain both promise regular payments — but the probability of actually receiving them is very different. Credit ratings exist to quantify this difference, giving investors a shorthand for the risk they're taking on.
Three firms dominate global credit rating: Moody's, Standard & Poor's (S&P), and Fitch. They assess the creditworthiness of bond issuers — governments, companies, municipalities — and assign letter grades. Institutional investors, pension funds, and many funds are legally required to hold only bonds above certain rating thresholds.
| S&P / Fitch | Moody's | Category |
|---|---|---|
| AAA | Aaa | Highest quality — near-zero default risk |
| AA | Aa | Very high quality |
| A | A | High quality — slightly more vulnerable |
| BBB | Baa | Investment grade — adequate protection |
| BB | Ba | Speculative / "junk" — below investment grade |
| B | B | Speculative — significant risk |
| CCC | Caa | Very high risk of default |
| D | C | In default |
The dividing line is BBB-/Baa3. Everything above is investment grade — considered safe enough for pension funds and insurance companies. Everything below is high yield, or colloquially "junk bonds." High yield bonds pay higher coupons to compensate for the extra default risk. The spread between high yield and government bond yields (the "junk spread") is one of the most closely watched gauges of market risk appetite.
A downgrade — cutting a rating — immediately raises borrowing costs for the issuer. Funds that can only hold investment-grade bonds must sell if a bond drops to junk ("fallen angel"), creating forced selling that pushes the price down further. A downgrade of a country's sovereign debt can trigger a financial crisis: Argentina, Greece, and Lebanon have all experienced this chain reaction.
An upgrade works in reverse — improving creditworthiness lowers yields, raises bond prices, and can trigger institutional buying.
The ratings agencies' credibility problem: in the 2008 financial crisis, mortgage-backed securities stuffed with near-worthless subprime loans were rated AAA by all three agencies. They failed catastrophically. The structural problem: issuers pay the rating agencies for their ratings — a clear conflict of interest. Ratings are a useful starting point, but sophisticated investors do their own credit analysis rather than outsourcing judgment entirely to agencies.
For UK investors: most retail investors don't buy individual corporate bonds — minimum trade sizes are often £100,000+. The more practical route is high-yield bond ETFs or investment-grade corporate bond ETFs, which give you diversified exposure to credit risk at low cost. iShares and Vanguard offer both in ISA-eligible formats.
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